There is a strong interest in flexible TFT back-panels (including insulation and hermetic sealing, if required) because of the strong interest in wearable electronics with high information content displays and/or sensors. The key challenge in this technology is the processing substrate. To produce good electrical performance, the processing substrate has to survive high temperature and all kinds of chemical treatments without deformation. Furthermore, the processing substrate has to remain flat for lithography purposes. No flexible substrate can achieve these requirements. The only solution is to mount the flexible TFT back-panel onto a rigid glass support using an adhesive for processing purposes and removing the flexible TFT back-panel from the glass support by de-bonding after the processing is complete. There are a lot of challenges in finding the adhesive material and the suitable flexible TFT back-panel that can maintain the necessary dimensional stability.
For most flexible substrates the coefficient of thermal expansion (CTE) is 50 to 100 ppm per degree Celsius, but for glass substrates the CTE is only a few ppm per degree Celsius. For temperature excursions of 300 degrees Celsius the deformation in plastic substrates can be as large as a few percent. Thus, to hold the shape when using a glass support with a flexible TFT back-panel there will be a strong stress to bend the glass. The thicker the material used in the TFT back-panels, the stronger the force tending to bend the glass support. For typical TFT back-panels including plastic of 100 to 200 microns thick (e.g. insulating layers), the force is so strong even the glass substrate is bent. Therefore, it is advantageous to use a plastic material that is as thin as possible. For handling purposes a plastic substrate added to the TFT back-panel has to be sufficiently thick to deal with the handling after the de-bonding.
The de-bonding process is also tedious and can require high temperature or laser illumination. Also, the de-bonding process can damage the back plane and may not be compatible with display/sensor device processing, such as the fabrication of OLED or organic photodiodes. For example, OLEDs or photodiodes cannot survive temperatures over 100 degrees Celsius. Even finished LCD cells cannot survive the high temperature that is required for de-bonding. Such limitations can force the de-bonding process to be carried out before the display/sensor device processing. However, if the flexible substrate/glass support is de-bonded before the display/sensor device processing, the display/sensor device processing has to be carried out on the flexible substrate, which is more difficult to handle. Because of these problems and others, it is very difficult to fabricate good flexible backplanes and flexible displays/sensors with high information content.
The advance of metal oxide thin film transistors (MOTFT) enables high performance TFTs to be made at lower temperatures. However, the decrease in performance and stability in MOTFETs is still an issue even for MOTFETs made at low temperature. Therefore, it is advantageous to be able to make flexible backplanes at high temperatures up to 300 degrees Celsius. Thus, the use of a rigid supporting element to hold a flexible substrate rigid during processing is still desirable.
In the prior art various attempts to fabricate flexible substrates on glass supports have been made. One example is described in U.S. Pat. No. 8,258,694, entitled “Method for Manufacturing Flexible Display Device Having an Insulating Overcoat and Flexible Display Device Having the Same”, issued Sep. 4, 2012 and a divisional thereof, U.S. Pat. No. 8,257,129. In this type of process, an insulating protection layer is formed on a rigid substrate (e.g. glass). Display elements are formed on the insulating protection layer and a flexible substrate is formed in an overlying relationship on the display elements. The rigid substrate is then removed by etching or the like. To perform the etching step, the material of the rigid substrate must have at least an etching selectivity 20 times greater than the insulating protection layer. It is important to note that the insulating protection layer does not stop the etching but is only etched at a much slower rate. That is to say, in all known prior art some of the insulating protection layer is removed in the process of etching the glass substrate. Since the amount removed affects the efficiency of the insulating protective layer (i.e. the protection provided by the layer) some compensation must be provided in advance. However, forming a thicker insulating protection layer in advance increases the stress on the glass substrate. Generally, many of these prior art fabrication methods have either been extremely difficult to use, usually because of the severe requirement for the selection of materials, or have failed completely because the etching step is too rigorous.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved process for fabricating a flexible TFT back-panel on a glass support.
It is another object of the present invention to provide a new and improved process for fabricating a flexible TFT back-panel on a glass support member wherein a step of etching the glass support member is greatly simplified in both the etching process and the selection of materials.
It is another object of the present invention to provide a new and improved process for fabricating a flexible TFT back-panel on a glass support member wherein a step of etching includes the use of reusable noble metals to greatly reduce the cost.
It is another object of the present invention to provide a new and improved flexible TFT back-panel on a glass support.